CONSTRAINTS ON TUNNEL VALLEY FORMATION FROM DEPTHS OF INCISION AND SEDIMENT FILLS

Multiple networks of tunnel valleys were formed beneath the Saginaw Lobe of the Laurentide Ice Sheet in Michigan, during advance to its LGM extent and its Late Wisconsin retreat. Here we focus on a regularly spaced (7-10 km) set of valleys in south-central Michigan that radiate from the center of the lobe in conformance to the presumed subglacial hydraulic gradient. These valleys can be subdivided into distal and proximal groups. Valleys in the distal set, which terminate at or near the Kalamazoo ice-marginal position, are deeply incised, filled with coarse gravel and sand, contain large eskers that fine upward in grain size, and overlie the subcrop of the Marshall Sandstone. High relief hummocky topography, kames, and ice-walled lake plains in this area indicate stagnation of a broad area behind the ice margin. The proximal valleys are open, less deeply incised, lack thick sediment fills, and overlie sedimentary rock units of lower permeability. These characteristics support a model in which subglacial drainage conduits are fixed in position and utilized for significant periods of time. The greater incision and thick, coarse sediment fills of the distal valleys suggest a greater contribution of meltwater derived from either surface melting and drainage to the bed near the margin, greater discharge of groundwater from the more permeable substrate into the conduits, or both. The possible role of catastrophic drainage events in the erosion of these valleys is unknown.